Memory devices using semiconductor elements are roughly classified into volatile memory devices that lose their memory content when power supply is stopped and nonvolatile memory devices that can hold their memory content even when power supply is stopped.
As a typical example of a volatile memory device, a dynamic random access memory (DRAM) is given. In a DRAM, a transistor included in a memory element is selected and an electric charge is accumulated in a capacitor, so that information is memorized.
Owing to the above-described principle, a charge in a capacitor is lost when information data is read in a DRAM; thus, it is necessary to perform rewriting of the information data, so that information is memorized again after the information data reading. In addition, there is leakage of current in a transistor included in a memory element so that a charge accumulated at an electrode of the capacitor is lost or a charge flows into the capacitor, even if the transistor is not selected to perform any operation, whereby data holding period is short. Therefore, it is necessary to perform rewriting in a predetermined cycle (refresh operation) and it is difficult to reduce power consumption satisfactorily. Further, since memory content is lost when the power is not supplied to the DRAM, another memory device using a magnetic material or an optical material is needed to store information for a long period.
As another example of a volatile memory device, a static random access memory (SRAM) is given. In an SRAM, memory content is stored using a circuit such as a flip flop, so that refresh operation is not needed. In view of this point, SRAM is advantageous over a DRAM. However, an inconvenient in that a cost per storage capacity becomes high because a circuit such as a flip flop is used. Further, in view of the point that memory content is lost when the power is not supplied, an SRAM is not superior to a DRAM.
As a typical example of a nonvolatile memory device, flash memory is given. A flash memory includes a floating gate between a gate electrode and a channel formation region in a transistor. A flash memory stores memory content by storing a charge in the floating gate, so that a data holding period is extremely long (semi-permanent), and thus has an advantage that refresh operation which is necessary in a volatile memory device is not needed (for example, see Patent Document 1).
However, in flash memory, there is a problem in that a memory element does not function after performing writing operations a predetermined number of times because a gate insulating layer included in the memory element is deteriorated by a tunnel current occurring each time a writing operation is performed. In order to relieve an adverse effect of this phenomenon, a method consisting in equalizing the number of writing operations between the memory elements is employed, for example. However, a complicated peripheral circuit is needed to apply this method. Further, even if such a method is employed, the basic problem of lifetime is not resolved. That is, a flash memory is unsuitable for applications in which information is rewritten with high frequency.
Further, high voltage is required to store a charge in the floating gate or to remove a charge in the floating gate. Furthermore, a relatively long time is required for storing or removing a charge and the speed of writing and erasing cannot easily be increased.